If "time" went back then forward again, would everything turn out the same?

[Windevoid]

If "time" went back then forward again, would everything turn out the same?

 

It may be a philosophical question or a quantum or classical one, but I just decided to put it here.

[ajb]

This reminds me of the Novikov self-consistency principle; one cannot create time travel paradoxes.

 

As an example, you could not go back in time and stop the Titanic from sinking, even if you warned the Captain. More than that, you were always part of the history of the Titanic sinking and did warn the Captain at the time, but did not prevent the sinking. You may have even have coursed the sinking!

[Windevoid]

This reminds me of the Novikov self-consistency principle; one cannot create time travel paradoxes.

 

As an example, you could not go back in time and stop the Titanic from sinking, even if you warned the Captain. More than that, you were always part of the history of the Titanic sinking and did warn the Captain at the time, but did not prevent the sinking. You may have even have coursed the sinking!

I'm not totally sure that is true.

 

You see, if you go back in time and push the iceberg away or stop and/or turn the ship before it is too late, you would prevent the sinking.

[Greg H.]

I'm not totally sure that is true.

 

You see, if you go back in time and push the iceberg away or stop and/or turn the ship before it is too late, you would prevent the sinking.

That's the point of the self-consistency principle: You cannot make changes to the past - you simply become part of the narrative resulting in the same outcome.

 

You turn the ship, and it rams into another part of the iceberg. You stop the ship, and the iceberg rams the ship. Or you inadvertently distract the crew running around shouting "Beware the iceberg" and they ram the berg because they were too busy dealing with you.

 

 

The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general relativity (solutions containing what are known as closed timelike curves). Stated simply, the Novikov consistency principle asserts that if an event exists that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero. In short, it says that it is impossible to create time paradoxes.

 

You can read more at Novikov Self-Consistency Principle

[Windevoid]

That's the point of the self-consistency principle: You cannot make changes to the past - you simply become part of the narrative resulting in the same outcome.

 

You turn the ship, and it rams into another part of the iceberg. You stop the ship, and the iceberg rams the ship. Or you inadvertently distract the crew running around shouting "Beware the iceberg" and they ram the berg because they were too busy dealing with you.

 

 

 

You can read more at Novikov Self-Consistency Principle

Perhaps the universe can change instead of this determinism that you are talking about.

And if you read any of my other posts, the universe seems to have paradoxes already.

[swansont]

There's also the problem of posing a scenario that isn't physically possible. If you violate the laws of physics, you can't rely on the laws of physics to tell you what happens.

[Windevoid]

There's also the problem of posing a scenario that isn't physically possible. If you violate the laws of physics, you can't rely on the laws of physics to tell you what happens.

I suppose you are talking about entropy and the arrow of time.

[swansont]

I suppose you are talking about entropy and the arrow of time.

 

That and time travel in general, at least of the sort you are proposing.

[Windevoid]

Link to topic:

 

"Alternative to Novikov self-consistency principle"

 

http://www.scienceforums.net/topic/78256-alternative-to-novikov-self-consistency-principle/

Edited August 24, 2013 by Windevoid

[Mr Monkeybat]

This reminds me of the Novikov self-consistency principle; one cannot create time travel paradoxes.

 

As an example, you could not go back in time and stop the Titanic from sinking, even if you warned the Captain. More than that, you were always part of the history of the Titanic sinking and did warn the Captain at the time, but did not prevent the sinking. You may have even have coursed the sinking!

Quantum physics says that an atom can be in two states at the same time. So perhaps after the the time travelers intervention the Titanic is both sunk and floating at the same time until the Earths wave function collapses due to an outside observer.

[pears]

Does time actually 'flow' in this way at all from the pov of physics? Isn't time just a 'space' for want of a better word, and the present just a point in that space? Sure time appears to 'flow' from an individual conscious mind perspective, but can it actually 'go' forward and backward like this?

[ajb]

Sure time appears to 'flow' from an individual conscious mind perspective, but can it actually 'go' forward and backward like this?

For time to just start running backwards does seem very unnatural. We have various notions of the arrow of time. At the microscopic level the laws of physics are invariant under changing the direction of time, however macroscopically time only seems to run forward.

 

We also have the related, but separate issue closed time like curves in general relativity.

[pears]

But does time actually 'run' at all? I understand there is an arrow of time which gives it a direction, but isn't time itself a plane through which things pass in a particular direction as opposed to something that flows over us? If things were to move backwards through it, if we were to move backwards through it, wouldn't our internal clock that gives us our sense of time, actually go in reverse? So would we even notice the difference? Trying to imagine time at first 'stopping' and then changing direction, twice, requires one to step outside of time while still maintaining a sense of time - i.e. my use of words such as first, and then - almost as if one steps from our finite space-time into eternal or absolute time. I'm unable to this idea of time stopping, and then running backwards, and then forwards, with any coherent meaning, at least in the realms of natural physics.

 

However, I could picture space-time as a sheet through which something, a point, (or indeed everything - a line) passes along it in the past to future direction, and then the point or line changes direction and moves backwards for a bit (without necessarily any awareness it has done so) and then moves forwards into a new branch of the sheet. The 'old' future (yes struggling with language here) is still there, it has not ceased to exist, but the point or line on the space-time sheet is now passing through a different plane. This is how I would visualise or express the idea in the OP.

 

I hope that made sense!

At the microscopic level the laws of physics are invariant under changing the direction of time, however macroscopically time only seems to run forward.

 

Does wave-function collapse give an arrow of time at the quantum level? That was hinted at at the wikipedia article.

 

We also have the related, but separate issue closed time like curves in general relativity.

 

 

I haven't heard of these. I'll look them up!

[ajb]

But does time actually 'run' at all?

We measure time marching on forwards.

 

What you describe next sounds like a space-time cut, that is splitting space-time into space and time in the context of general relativity. The problem is you cannot always do this globally in a meaningful way. Deciding globally "now" is not easy in general.

 

Locally this us fine, we can for massive particles parametrise their path in space-time with their proper time. That is the time as measured by a clock moving along that path.

 

 

Does wave-function collapse give an arrow of time at the quantum level? That was hinted at at the wikipedia article.

Yes, this is the quantum mechanical arrow of time. The time evolution of quantum mechanics is time reversible, but wave function collapse is not. In the modern view this is really a consequence of the thermodynamic arrow of time. The idea roughly is that the universe gets more and more disordered and this gives an arrow of time. Look up entropy and the second law of thermodynamics.

 

Thermodynamics is pretty cool. It generally does not care about the details of the physics at the level of individual constituents, but rather gives general principals that all collections of objects must obey. Because of this, most of the other arrows of time are really equivalent to the thermodynamic arrow.

[pears]

We measure time marching on forwards.

 

 

Yes - we, the observer, experience time like it's passing over us. But 'it' does not necessarily 'flow'. More we move through it. I don't know - maybe it's just a you say to-may-to I say to-mar-to thing.

 

What you describe next sounds like a space-time cut, that is splitting space-time into space and time in the context of general relativity. The problem is you cannot always do this globally in a meaningful way. Deciding globally "now" is not easy in general.

 

 

True. I was just trying to visualise the idea as expressed in the OP as I struggle with the idea of time flowing, stopping, then running forwards again, without somehow stepping outside of time into another type of more absolute time. I suppose what I'm really trying to say is yes, everything would turn out the same, because I don't view time in that sense, as running backwards and forwards like that.

[ajb]

Yes - we, the observer, experience time like it's passing over us. But 'it' does not necessarily 'flow'. More we move through it. I don't know - maybe it's just a you say to-may-to I say to-mar-to thing.

 

It is more like we flow through space-time and time is the parameter that defines this flow. Basically for classical massive particles the proper time can be used to parameterise curves c: R -> M that describe the motion of the particles.